lib/oe/package-manager: allow including self in create_packages_dir

This function is typically used to construct a limited feed for image
creation, but there are other cases when you might want a limited feed
and include the current recipe's packages in it.

To ensure that existing behaviour is preserved, add a boolean to control
this behaviour and default it to False.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>